The present invention relates to a photomask which is used for the fabrication of integrated circuits with extremely high density such as VLSI and ULSI, and a method for producing the same. Particularly, the present invention relates to a phase shift photomask having a transparent film which serves as a phase shifter useful for creating minute patterns with high density, and a method for producing the same.
Semiconductor integrated circuits such as IC, LSI and VLSI are conventionally produced by repeating the so-called lithographic process in which after a resist layer is coated onto a substrate such as a Si wafer and then exposed image-wise to light by a stepper or the like, development and etching are conducted to obtain a desired resist pattern.
As semiconductor integrated circuits become more complicated and have higher integration, a photomask called a reticle, used in the above lithographic process tends to have higher density.
It is demanded that the line width of a device pattern created by using such a reticle be narrower: for instance, a line width of 1.2 .mu.m for 1 Mbit DRAM, 0.8 .mu.m for 4 Mbit DRAM, and 0.5 .mu.m for 16 Mbit DRAM. In order to respond to such a demand, various exposure methods are now being studied.
For example, a pattern having a line width of 0.35 .mu.m is necessary for a device of a 64 Mbit DRAM class. In a conventional stepper exposure method using a reticle, this line width is a resolution limit of a resist pattern. In order to overcome this limit, phase shift lithography using a phase shift reticle has been proposed as shown, for instance, in Japanese Laid-Open Patent Publication No. 173744/1983 and Japanese Patent Publication No. 59296/1987. As will be described later in detail, this is a technique of improving the contrast and resolving power of a projected image by controlling the phase shift of light which passes through a reticle.
The phase shift photomasks are classified into some types such as of the alternating type with a shifter placed at the upper part thereof, the alternating type with a shifter placed at the lower part thereof, and the attenuated type as described later. However, the production of any of the above photomasks comprises the step of forming a transparent film serving as a phase shifter at any proper position in the layers needed for a phase shift photomask formed on a substrate.
One of the methods for forming this transparent film serving as a phase shifter is as follows: SOG (Spin-On-Glass) (an application-type SiO.sub.2 coating layer) is dropped on the upper surface of a substrate, spread over the entire rectangular surface of the substrate by rotating the substrate at high speed, utilizing the centrifugal force which acts on the SOG, and then dried by evaporating any solvent contained therein.
In the above process, the SOG protuberates at the periphery of the substrate because the boundary conditions are different. Such a protuberance cracks or peels off the substrate when baked, so that it is the source of dust in the production of a photomask.
There is also a case where a sputtering method is employed for forming the transparent film serving as a phase shifter. In this case, the transparent film serving as a phase shifter is formed even in a chamferred area, the peripheral area of a substrate. The chamferred surface and periphery of the substrate have fine micro-cracks which are produced during a chamfering process, so that the adhesion of the transparent film to the substrate in the peripheral area is weaker than that to the substrate in the central area. For this reason, there is a phenomenon that the transparent film formed in the peripheral area peels off the substrate as fine particles during the production of a photomask or the use thereof.
In general, therefore, the manner in which SOG in the peripheral area of a substrate is wiped out, before SOG is baked, with a cloth or the like infiltrated with an organic solvent has been adopted in the SOG-coating method; and the manner in which hydrofluoric acid or an aqueous strong alkaline solution is used for the removal of SOG in the peripheral area of a substrate has been adopted after SOG is baked in the SOG-coating method, or in the SOG-sputtering method.
However, there are the following problems with the conventional manners for removing a material for the transparent film serving as a phase shifter at its peripheral portion. In the manner in which the material for a shifter at its peripheral portion is wiped out with a cloth infiltrated with an organic solvent, photomasks are wiped one by one, so that fine particles of the material for a shifter are deposited to the substrate unless the condition of the cloth is strictly handled. Thus, this manner causes many problems on the quality and handling.
Further, regarding the manner in which the material for a shifter at its peripheral portion is removed by using hydrofluoric acid or an aqueous strong alkaline solution, these chemicals are poisons as are commonly known, so that they are inconvenient in handling. Moreover, in this manner, the peripheral area of a substrate to be treated is dipped in hydrofluoric acid or an aqueous strong alkaline solution. Therefore, the portion of the transparent film serving as a phase shifter to be removed cannot be stably controlled due to the rise of the aqueous solution to a level higher than the dipped surface of the substrate, rippling of the surface of the aqueous solution, deterioration of the solution, or the like.